1. Field of the Invention
The present invention relates to an improved method and apparatus for deflecting light using a rotary polyhedral mirror.
2. Description of the Prior Art
Various devices and methods are known in the art for scanning a beam of light, such as laser light, over a record medium for the purpose of reading and recording information. A rotary polyhedral mirror is one kind of light deflector used in such devices. Unfortunately, however, even if the rotary polyhedral mirror is made with the highest degree of precision, there will still be an error in parallelism between its rotary axis and the mirror surfaces. Since, an error in parallelism causes the beam of light to be deviated in a direction which is at right angles to the scanning direction, the loci of scanning lines due to the mirror surfaces of the rotary polyhedral mirror will not coincide. The longer the length of the light path from the mirror to the scanning surface, the greater the deviation of the scanning lines as a result of the error in parallelism.
Several systems have been proposed to optically correct the latter problem. For example, according to the invention disclosed in the specification of Japanese Patent Laid-Open No. 33,624/ 1972, the amount of correction needed to compensate for errors in parallelism in the mirror surfaces of the rotary polyhedral mirror is pre-measured and stored in a memory. Thereafter, in synchronism with rotation of the rotary polyhedral mirror, a separately installed light deflector is driven by a signal from the memory to thereby eliminate the deviation of the scanning line of the light beam. One disadvantage of the latter system is that it requires a complicated device for driving the light deflector in response to the signal from the memory.
Further, in the invention disclosed in the specification of Japanese Patent Laid-Open No. 98,844/1973, corresponding to Fleischer U.S. Pat. No. 3,750,189, issued July 31, 1973, two cylindrical lenses are employed to form an optical system, wherein a deflection point and a scanning surface screen are an object point and an image point, to thereby remove the deviation of the scanning line due to the error in parallelism. This system has a disadvantage, in which an error in parallelism is corrected only at the image point.